1. Field of the Invention
The present invention relates to a developing unit included in an image forming apparatus of an electronic photography type, and more particularly to a mechanism for feeding or exchanging developing powder faster and a mechanism for adjusting a toner density sensor when the developing powder is fed into the developing unit, the mechanisms being provided in the developing unit.
2. Description of the Related Art
The inventors of the present application know a mechanism for feeding developing powder to a developing unit. The known mechanism includes a toner density sensor and is constructed so that when the sensor senses the absence of developing powder being removed in the developing unit, a rotary system for removing the waste developing powder is deactivated and a feeding unit is activated in response to the sensing signal, for feeding a necessary amount of new developing powder into the developing unit. This mechanism has been disclosed in Japanese Patent Lying Open No. 61-39061.
In the known mechanism, the rotary system such as rollers and screws provided in the developing unit is driven at a constant speed needed in a developing process. The same speed is maintained in feeding new developing powder, exhausting waste developing powder, and developing an image. Further, the feeding unit is kept driven until the necessary amount of new developing powder is charged in the developing unit.
Hence, the known mechanism may has a longer set time for exchanging the developing powder than the actually consuming time, because the time is set in consideration of the variety of the charging operation and the state of the developing powder. During the time, the image forming apparatus has to be kept untouched for a maintenance worker or a user. It means that the time when the mechanism is continued to be driven wastefully after supplying of the developing powder is a wasteful time for the maintenance worker or the user. The wasteful time results in making the operation efficiency lower.
Further, when doing the maintenance work, a photosensitive drum may be removed from an image forming apparatus in advance. When the developing powder is exchanged in this state, the developing powder may be splashed from the exposed surface of a magnet roller being driven for exchanging operation. The fall of the carrier and the splash of the toner result in soiling the inside of the image forming apparatus. It means that the splash of the developing powder has an adverse effect on the quality of the resulting image.
On the other hand, when feeding new developing powder, there are areas on which the developing powder is attracted and areas on which no developing powder is attracted on the surface of the magnet roller in rotation. Those areas are variable on the surface of the magnet roller according to the feeding amount of the developing powder. This may bring about the splash of toner and carrier inside of the image forming apparatus.